Electroplating process



Patented Apr. 2 7, .1937

V UNITED STATES ELECTROPLATING PROCESS Floyd F. opunger, Niagara Falls,N. r., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del.,a corporation of Delaware No Drawing. Application January 31, 1934,

Serial No. 709,171

12 Claims. (Cl. 204-17) This invention relates to electroplating metalsand more particularly to a method of producing bright, smooth coatingsof electroplated tin, cadmium, zinc and their alloys.

5 Electroplated coatings of tin, cadmium, zinc and alloys of thesemetals when taken directly from the electroplating bath ordinarily havea dull finish and are very susceptible to fingerstaining and totarnishing. For most purposes in a bright, non-tarnishing finish isdesirable. Or-

dinarily such electroplated coating is brightened by buffing andpolishing; this, however, does not substantially reduce the tendency totarnish.

Coatings of these metals, having a bright ap- 1; pearance, also may beobtained by dipping the metal articles to be coated in a molten bath ofthe coating metal. In order to obtain a smooth, uniform and brightsurface by this method it is usually necessary to further treat thecoated articles, e. g. by rolling, wiping or other treatment to removeexcess metal and smooth the coatings. While this hot dipping method. ofcoating metals is capable of producing a bright, smooth surface, it hasa number of disadvantages. In the first place, when iron or steelarticles are thus coated, careful and tedious preparation 1 of the metalsurface to remove oxide and other impurities is necessary before themetal is hot dipped in order to obtain a smooth adherent coat. Thesepreliminary steps include pickling operations and annealing.Furthermore, metals which are coated by hot dipping have a relativelythin layer of coated metal because of the fact that the molten metal hasa relatively low viscosity and when the article is removed from the hotdipping bath excess metal drains ofi, leaving a relatively thin coatingon the article. The thickness of the coating is further reduced when awiping or rolling operation is used. Hot dipping methods are also ingeneral not readily applicable to odd shaped or deeply recessedarticles. Frequently great difliculty is encountered in hot dippingmethods because the metal to be v coated tends to dissolve in thecoating metal so 41-, that the coating metal bath becomes rapidlycontaminated.

In many cases an electroplating process is preferable for coating metalsbecause less preparation of the metal prior to plating is required and;o a thicker coating may be obtained. However, heretofore no entirelysatisfactory process has been proposed for producing brightelectroplated coatings of soft metals such as tin, cadmium or zinc.While bright electroplated coatings may be 35 obtained by plating undercertain carefully controlled conditions or by buffing and polishing,such plates are very easily tarnished and flngerstained. While it ispossible to obtain a bright surface on a plated article by bufiing andpolishing without undue removal of coated metal, it is 5 difiicult andusually impractical to obtain a uniform bright coat on articles ofirregular shape because of the difficulty of buffing and polishing deepdepressions and irregular surfaces.

An object of the present invention is to produce smooth, bright,electroplated coatings of fusible metals such as tin, zinc, cadmium ortheir alloys Without the use of mechanical polishing operations, bathaddition agents, etc. A further object is to produce such coatings oniron or steel. A still further object is to produce electroplatedcoatings of tin, cadmium, zinc or their alloys on iron or steel whichcoatings have superior adherence to the base metal and increasedcorrosion resistance. Other objects will appear from the followingdescription of my invention.

The above objects are accomplished by electroplating metallic articleswith metals fusible below the melting point of the base and subjectingthe electroplated articles to a heat treatment under substantiallynon-oxidizing conditions at a temperature above the melting point of thecoated metal. Preferably the coated metal is heated to a temperatureslightly above its melting point for a short time, for example 10 toseconds. My invention also comprises a novel method of pickling iron orsteel articles prior to electroplating as hereinafter described whichresults in a superior electroplated coating. By

these novel methods I have obtained smooth, bright coatings of zinc,cadmium, tin and their alloys which are characterized by having a lowdegree of porosity.

I am aware that it has been proposed to improve electroplated coatingsby the application of heat. However, so far as I am aware, heretofore ithas not been proposed to electroplate metals and subsequently heat theelectroplated surface to a temperature above the melting, point undersubstantially non-oxidizing conditions to 5 obtain the smooth, bright,oxide-free coating which characterizes my invention. r

I have discovered that when heavy base metals such as iron, copper orbrass are electroplated with metals having a melting point lower thanthe base metal, such as steel, zinc, cadmium, tin' or their alloys andthe electroplated article is subsequently heated under substantiallynonoxidizing conditions to a temperature above the 7 melting point ofthe base metal, a bright smooth Y plate is obtained which has a lowdegree of porosity. Such plate is similar in appearance to that obtainedby the best methods of hot dipping or hot galvanizing and may be made ofa much greater thickness. I have further discovered that such basemetals, especially iron or steel may be plated with improved results ifprior to plating the metal is treated with astrong acid to such extentthat a distinct and uniform etching corrosion of the surface isobtained, followed by an alkaline treatment. With such pretreatment, theadherence of the plated coating is improved and the porosity of theplate is materially decreased. I have further found that when suchpretreated metal is plated with tin, cadmium, zinc or their alloys andthe plated coating is heat treated to a temperature above its meltingpoint as described above, the resulting plate is smoother and moreuniform than is the case when the base metal is not so etched and alkalitreated prior to plating.

The above-mentioned etching prior to electroplating is especially usefulin obtaining adherent, non-porous coatings of tin or the rust-resistingmetals on cold-rolled sheet steel. Cold-rolled steel ordinarily has abright, smooth surface. When bright, cold-rolled steel is electroplatedfor example with tin, the plated metal has relatively poor adherence andcorrosion resistance. However, if the steel is previously treated withacid and alkali as described above, the electroplated tin has goodadherence to the base and the corrosion resistance is markedly improved,without increasing the depth of the electroplated layer. These improvedresults are especially marked when the electroplated coating is in theneighborhood of 0.0001 inch thick or heavier.

One method of practicing my invention will be illustrated by referenceto the production of a smooth, bright coating of tin or tin alloy onsteel. The surface of the steel after suitably cleaning to remove greaseand dirt, is first treated by immersion in a strong acid solutionpreferably at an elevated temperature, e. g. 100-200 F., until thesurface of the metal'is distinctly etched so that a uniformly etched orcorroded surface is plainly visible to the unaided eye. The steel isthen treated with an alkaline solution, preferably by making the steelthe anode in an alkaline cyanide bath and applying a moderate electriccurrent for a minute or two. The treated steel is electroplated with tinor tin alloy by any suitable method to produce a tin layer of thedesired thickness. I prefer to electroplate from an alkaline'solution,especially by the methods described in U. S. Patents 1,841,978 and1,919,000. These or similar methods may be used for plating tin or tinalloys on the treated iron or steel.

After the desired thickness of tin or tin alloy has been plated on thesteel, the article is heated under substantially non-oxidizingconditions to a temperature above the melting point of the tin or tinalloy coating. If the coating is of substantially pure tin (which meltsat about 450 F.), heating maybe to a temperature of about 460 to 500 F.for a period of ten to thirty seconds. I

Various known means of heating under nonoxidizing conditions may beutilized in order to carry out my invention. For some work and withrelatively low melting coatings I have found a bath of molten tallow tobe suitable; for other pieces and especially for continuous operation,

Example 1 Samples of cold rolled sheet steel (automobile body stock)were first freed from grease and dirt by cleaning in an alkalineelectrolytic cleaner containing OzJsal. Sodium cyanide 2 Caustic so 2Trisodium phosphate 4 Temperature of solution-160-180 F. making itcathode and anode alternately; until entirely free from grease and dirt.

The samples then were pickled in a sulfuric acid solution containing byvolume of sulfuric acid (66 B.) at a temperature of 70-90 C. for aperiod of 4 minutes. Aside from the removal of oxide or scale, thepickling was continued until the rolled surface layer was removed and adistinct etching was visible to the naked eye.

The samples then were water rinsed and further treated electrolyticallyin a sodium cyanide solution containing 4 oz./gal. of NaCN at 140-160F., by making it the anode at 100 A/SF for 1 minute. After thistreatment the samples again were water rinsed and then tin plated in analkaline tin solution containing:

V 0z./gal. Sodium stannate 12 Caustic soda 1 Sodium acetate 2 Hydrogenperoxide y,

(100 vol.)

Data

Anode Straits tin Ratio of anode to cathode area 3 to 1 Cathode currentdensity-+10 to 50 A/SF E. M. F 4.0 to 6.0 volts Temperature of solution60 to 80 C. Time of plating 3 to minutes Thicknesses of 0.00005" to0.0004" of tin were produced.

Following the plating, the samples were rinsed in cold and hot water anddried. The dry; samples were immersed for 10 to 20 seconds in a moltentallow bath at a temperature of 240-260 C. This treatment producedbright, smooth coatings of tin similar in appearance and hardness tothat produced by hot tinning processes.

Eabample 2 Samples of cold rolled sheet steel were cleaned and thentreated with acid and cyanide solution as described in Example 1. Thetreated samples were plated in an acid tin solution made up as follows:

Oz./gal. Sodium stannate 6 Sulfuric acid 16 Cresylic acid 1 Glue V9,018,868 3 Data Example 7. -"w n Strait-Jilin Samples of sheet brassand copper were first Ratioof amde to catmde area 3 to I cleaned bymaking them cathodes for 3 to 5 Catlmde current density 6 A/SF minutesat 160 to 180 F. in an alkaline solution E. M. F 2.0 to 4.0 voltscontaining. Temperature of solution oz Mal I Normal room temperaturesodium cyanide 2 Time of plating 3-0 to minutes msodmm f 2 Thickness ofcoatings 0.00005" to 0.0004" Caustic soda 4 Following the plating, thesamples were heated I in a w a as described in Example 1, with iii/2t?iitif witii tiniitftte iiniiefwiifl Substantially identicalresultspickled at 140 to 160 F. for 2 to 4 minutes in a solutioncontaining: Example 3 Part by volume Cold rolled sheet steel sampleswere treated. Water 1 plated and heat treated as described 111 Exa p eMuriatic acid 1 t f gg g ig f g acid solution was The samples then werewater rinsed and immersed for 3 minutes at 140 F. in a solution HCl Da tby vo ume 1 containing 4 oz./gal. of sodium cyanide. The Water part byvo um 1 treated samples then were plated and heat treated Temperature ofsolution C- 70 to 90 as in Example 1 Time of immersion minutes 2 to 5Example 8 x mp 4 Samples of sheet brass and copper, cleaned as Theprocedure of Example 3 was repeated described in Example 7, were treatedin the folcept that the following acid solution was used to lowingsolutim' etch the steel: v o Cc./L

HNO; (cone) 1 part by volume Stnfuric .acid F 53o Sulfuric (cone) 1Nitric acid (42 Be.) 160 Water 4 Hydrochloric acid (24 B.) 0.75Temperature Normal room temperature 'Water f 320 Time of immersionseconds to 2 minutes After the acid treatment, the samples were immersedin a sodium cyanide solution contain- Example 5 ing 4 oz./gal. of sodiumcyanide for 3 minutes Samples of hot rolled steel were first freed from1 a temperature of Th mpl h grease and dirt by cleaning at 160 to 180 F.in were plated and heat treated as described in an alkaline electrolyticcleaner containing: p e With similar results.

ozJgag Example 9 22: :32 SgSE E LIIIIII 2 Pieces of cold-rolled sheetsteel were first Trisodium phosphate L 4 cleaned to remove grease anddirt as in Exampie 1. .They were then plated without previous The workwas made cathode and anode alternateacid etching or alkali treatment ina tin 801w 1y until entirely free from grease and dirttion similar tothat of Example 1 to which had The samples were then pickled in asulfuric been added about i f cadmium oxide acid olu i containing byvolume of S111- dissolved in sodium cyanide solution. The plated furicacid at a temperature of to coatings had a thickness of 0.0001" to0.0004". 90 for a period of 1 to 3 minutes 50 that an fire Afterplating, the coatings were treated in molten scale was removed andetching was plainly vistallow at about C Evidenceof the preslble to t enaked y ence of cadmium in the coating was presented I The samp s w e ewater rinsed and by the fact that the coatings melted below 231 thertreated in a warm sodium cyanide solution 0,, th melting point of ticontaining 8 oz./gal. of NaCN at 140 to 160 E, Bright deposits ofcadmium-tin alloys were by making them anodes at a current'denslty ofproduced in all cases by the heat treatment. 50 to 100 A/SF for 1minute. After this-treat- However, the coatings were lumpy, due to thement, the samples were plated and heat treated omission of the acid etchand alkali treatment.

as in Example 1, with substantially the same resuits- Example Example 6Pieces of cold-rolled sheet steel were degreased In place of the anodiccyanide treatment men- 2 32122 1 353% g -3:5 :6 2; 2 tioned in theforegoing examples, sheet steel 0 0 0 2 4 samples, after being etched invarious acid solu- 32:23: 5 g gg fg z f 5225 :2: 3: $8, 22:::gfigggiggggfiigf treated anodically in a sodium cyanide solution i ozlgal. (6 oz./gal.) at 140 to 160 F., using a current Sodium hydroxide 2density of 50 to 100 A/SF for 1 minute. After sodium carbonate v 4 waterrinsing the pieces were then plated in the tin-cadmium bath described inExample 9. Upon The anodic treatment was continued for 1 to 2 ttreatment in tanow as described in Exanb minutes at a c e density of 50to 100 pie 9, adherent, bright coatings of cadmium-tin The samples werethen plated and heat treated alloy were obtained. These coatings weresmooth as in Example 1, with the same results. and entirely free fromlumps.

Example 11 Pieces of cold rolled sheet steel were degreased as describedin Example 1. They were then acid etched as described in Example 13.Following this the same anodic treatment as in Example 13 was used.

The pieces were then plated with tin as described in Example 1 to athickness of 0.0001" to 0.0003". Followingthis, they were plated withcadmium from a solution containing:

Oz./ gal. Cadmium oxide 4 Sodium cyanide 10 Sodium hydroxide 1 Data.

Anodes Cadmium Cathode current density 20 to 30 H/SF Temperature normalroom temperature Coatings of cadmium 0.0001" to 0.0003" thick wereapplied to the previously plated tin coating so that the total thicknessof the coatings varied between 0.0002 and 0.00006". These coatings werethen heat treated in tallow at 200 to 300 C.

Adherent bright coatings of tin-cadmium alloy, free from lumps or runswere obtained.

Emample 12 Pieces of cold rolled steel were prepared and plated with0.0001 to 0.0003" of tin as described in Example 11. Following the tinplating, the samples were zinc plated in a solution containing:

Oz./gal. Zinc cyanide 5 Sodium cyanide 3 Sodium hydroxide 4 Data Anodess Zinc Cathode current density 20 to 30 H/SF Temperature 40 to 50 C.

The tin plated pieces were zinc plated with 0.0001" to 0.0002" of zinc.

Upon being heat treated in tallow at 200 C. to 300 C., adherent, brightcoatings of tin-zinc alloy were obtained. The coatings were free fromlumps and other imperfections.

Example 13 Pieces of hot rolled sheet steel were degreased in analkaline electrolytic cleaner and than acidpiokled to remove rust andfire scale until a clean surface was obtained. Following the pickling,the pieces were immersed in a NaCN solution (6 oz./ gal.) and cleanedanodically for 1 minute using 50 to 100 A/SF. The samples were thenplated as described in Example 11 with 0.0002" to 0.0003" of cadmium.The cadmium plated pieces were then plated in a zinc solution asdescribed in Example 12 with 0.0001" to 0.0002" of zinc. The doubleplated pieces were then treated in tallow at 260 to 300 C. Adherentbright coatings of cadmiumzinc alloy were obtained free from lumps andother imperfections.

As mentioned above, in practicing my process I prefer to treat the metalto be plated, especially iron or steel, first with a strong acid undersuch conditions and for such length of time that a decided visibleetching corrosion of the metal is obtained and then to treat the metalwith an alkaline solution. This method of pretreatment of iron and steelmarkedly decreases the .po-

rosity of the electroplated coating and hence increases the corrosionreaistance of the plate. This effect is not limited to the low-meltinlmetals such as tin, zinc or cadmium; for example, I have found that thecorrosion resistance of nickel plate is improved by this pre-treatment.Furthermore, when it is desired to subsequently heat treat theelectroplated coating by the herein described method, this preliminaryetching and alkaline treatment assists in producing a smooth coating ofthe final product, especially when the electroplated coating isrelatively thick. If such relatively thick coatings are heat treatedsubsequent to plating without this preliminary treatment, the finalcoating sometimes has certain irregularities or lumpiness, apparentlycaused by an uneven flow of the molten electroplated coating during theheat treatment and/or a failure of the plated metal to adhere well tothe base metal. Also, if the metal is thus etched but the alkalinetreatment is omitted, the desired satisfactory results will not beobtained. I have found that when my preliminary treatment is used, suchirregularities may be substantially entirely eliminated.

The herein described preliminary etching and alkaline treatment issuitable for treating various heavy metals that are to be electroplated,e. g. steel, cast iron, brass and copper. It is especially advantageouswhen it is desired to produce bright coatings on cold rolled steel orcast iron by electroplating followed by heat treatment, as hereindescribed. The tendency for the plated metal to fail to adhere properlyto the base metal and form an irregular or lumpy coating during the heattreatment is most marked when the base metal is cold-rolled steel. Forthis reason, my preliminary acid etching and alkali treatments usuallyare necessary in order to produce a good coating on such steel by myinvention. In the case of hot-rolled steel and the non-ferrous metals,the etching and alkali treatments often may be omitted or replaced bythe usual cleaning operations with more or less satisfactory resiflts.However, it generally is preferable to use the etching and alkalitreatment, since it tends to improve the quality of the plate inany-case.

The extent to which the base metal should be acid etched during thepreliminary treatment may vary, depending on the nature of the basemetal That is, in some cases, the minimum etching may be suflicient forthe best results. The proper minimum time for treatment in the etchingacid may be readily determined by simple trials, but usually the etchingis carried on till a visible appearance of etching is noted.

Various acidic materials capable of corroding iron, steel or other basemetals are suitable for the preliminary etching step in accordance withmy invention. For example, sulfuric acid, hydrochloric acid, nitric acidor mixtures of these acids may be utilized. I prefer to use sulfuricacid in concentrations of 15 to 30% by weight. If desired, the acid maybe used as electrolyte and the article to be etched made the anode in anelectrolytic system. However, this electrolytic method of etchingordinarily offers little or no advantage over simple immersion in theacid solution. The time and temperature of the etching operation as wellas the concentration of the acid in the solution may be varied over awide range, provided that the operation-is continued over such length oftime as is necessary to produce the required etching effect. I pre- 2normal to 6 normal strength and carry out the etching operation at anelevated temperature,

for example 100 to 200 F.

The time required to etch the metal with acid.

and/or the type of acid or acid mixture required may vary depending onthe nature of the metal. The kind of acid required to corrode a .dependpartly upon the constitution of the steel and partly on the heattreatment and mechanical working to which it has been subjected. Thebest method of etching a given sample may easily be determined by simpletrials with various acids or mixtures at various temperatures. When itis desired to' produce a mirror-like plate by the herein described heattreatment after plating, the steel should not be etched too drastically.However, even when the steel is drastically etched, the heat treatmentsubsequent to plating produces a distinct brightening and a smooth,dense plate.

Various alkaline aqueous solutions are suitable for the alkali treatmentfollowing the above described etching operation. I have found that ingeneral the best results are obtained when the alkaline bath containssubstantial amounts of an alkali metal cyanide; if desired, a solutionof alkali metal cyanide alone may be used or sodium hydroxide or otheralkaline materials may be added. I prefer to use a cyanide solutioncontaining 2 to 10 oz; per gal. of sodium cyanide as an electrolyticcleaning bath, wherein the work to be treated is preferably made theanode and using an anode'current density of up to around 100 amps. persq. ft. The best results generally are secured by keeping the bath hot,e. g.. at a temperature of 140 to 160 F. and using an anode currentdensity of 50 to 80 amps.

.per sq. ft. Various other alkaline solutions may -be used for suchelectrolytic treatment.

The time of treatment may be varied widely; ordinarily to 3 minutes issuflicient when the electrolytic method is used- If the alkaline bath isused without the aid of the electric current, a somewhat longer time oftreatment usually will be required, e. g. up to around 30 minutes.

As mentioned above, my process is suitable for coating metals withalloys of tin, cadmium, or zinc as well as the pure metals. Variousother relatively fusible metals or alloys, e. g. lead or lead alloys,may be utilized in my invention. The alloy coatings may be obtained byelectroplating the alloy from electrolytes containing salts of alloyconstituents. An alloy coating also may be obtained in accordance withmy inventibn by plating out the metals to be alloyed in two or moreseparate layers and'then subjecting the electroplated article to theabove described heat treatment under non-oxidizing conditions. For

' example, an article may be plated with a layer of tin and then with alayer of cadmium or the tin may be deposited on the cadmium plate andthe article thus double plated subjected to a heat treatment undernon-oxidizing conditions at a temperature slightly above the meltingpoint of the resulting tin-cadmium alloy. Likewise, other metals may bealloyed by this method with tin, cadmium or zinc; for example,- atin-copper alloy may be made by first plating the article with tin andthen with a light copper plate and subjecting the electroplated articlesto the above described heat treatment. In this case, in order to producea tin-copper alloy at the surface, it

is essential that the copper be plated over the tin. If thetin is platedover the copper, I have found that" complete alloying of the copper andtin cannot be obtained without the use of a heat treatment atat'emperature far above the melting point of tin. Such high temperaturesare in many cases impracticable to use because of the effect they mayhave on the base metal. Hence in general when it is desired to producean alloy of tin, cadmium or zinc with another metal such as copper orsilver, which has a markedly higher melting point, by means of themultiple plating method, I prefer to plate the higher melting metal asthe exterior coat prior to heat treating.

Various known methods of heat treating under non-oxidizing atmospheremay be used; I prefer to use a bath of hot tallow, or other high boilingliquids, fused salts or the like, or a furnace which is continuouslyprovided with an atmosphere of oxygen-free hydrogen. Obviously thesepreferred methods may be modified by using other suitable non-oxidizingliquidor gaseous media. v

An advantage of my herein described inven-- tion is that it results in asmooth, bright plated coating which has superior corrosion resistantproperties. The coating has the density, hardness, low degree ofporosity and bright appearance which are characteristic of hot dippedcoatings and furthermore, my coatings may be made of greater thicknessthan by hot dipping methods. Also, the step of heat treating subsequentto plating in accordance with my invention simplifies the electroplatingoperation, since it eliminates the need of originally obtaining plateshaving the best .color and general appearance. The heat treatmentbrightens the dullest plates substantially as well as the brighter ones.Hence it is necessary only to regulate the electroplating bath to obtainplating suitable for heat treatment.

Furthermorevmy improved method of treating metal prior to plating isadvantageous not only in producing superior coatings by heat treatingsubsequent to plating, but also improves the electroplate itself asregards adherence and corrosion resistance. Hence,- improved results maybe obtained by this method without heat treating subsequent to plating.

I claim: 1 i

1,. A process for electroplating a metal comprising treating said metalin an acid solution until the metal is visibly etched;\thereaftersubjecting said metal to an anodic treatment in an alkaline solution,electroplating the treated metal with a metal or alloy having a meltingpoint below that of said treated metal and subsequently heating theelectroplated surface under substantially non-oxidizing conditions to atemperature above the melting point of the electroplated coating.

2. A process for electroplating steel comprising treating said steel ina 2 to 6 normal acid solution containing at least one of the acids ofthe group sulfuric acid, hydrochloric acid and nitric acid until themetal is visibly etched, thereafter subjecting said metal to an anodictreatment in an alkali metal cyanide solution and electroplattiallynon-oxidizing conditions to a temperature of 460 to 500 F. i

3. A process for electroplating cold-worked steel comprising treatingsaid steel in a 2 to 6 normal acid solution containing at least ofsulfuric acid until the metal is visibly etched, thereafter subjectingsaid metal to an anodic treatment in an alkali metal cyanide solutionand electroplating the treated metal with tin and subsequently heatingthe electroplated surface under substantially non-oxidizing conditionsto a temperature of 460 to 500 F. for 10 to seconds.

4. A process for electroplating cold rolled steel comprising treatingsaid steel in an acid solution containing at least one of the acids ofthe group sulfuric acid, hydrochloric acid and nitric acid until saidsteel is visibly etched, thereafter subjecting said metal to an anodictreatment in an alkali metal cyanide solution and electroplating thetreated metal with a tin coating and immersing the electroplated metalin a hot, substantially non-oxidizing liquid, whereby said tin coatingis fused to form a smooth, bright coating.

5. A process for electroplating steel comprising treating steel in anacid solution until the steelis visibly etched, thereafter subjectingsaid steel to an anodic treatment in an alkaline solu--' tion,electroplating the treated steel with a metal or alloy having a meltingpoint lower than that of said steel and subsequently heating theelectroplated surface under substantially non-oxidizing conditions to 'atemperature above the melting point of the electroplating coating.

6. A process for electroplating steel comprising treating steel in anacid solution until the steel is visibly etched, thereafter subjectingsaid steel to an anodic treatment in an alkaline solution,electroplating the treated steel with tin and subsequently heating theelectroplated surface 40 under substantially non-oxidizing conditionstoa temperature above the melting point of the electroplated coating.

- '7. A process for electroplating cold-worked steel comprising treatingsaid steel in an acid solution until the steel is visibly etched,thereafter subjecting said steel to an anodic treatment in an alkalinesolution, electroplatingthe treated steel with tin and subsequentlyheating the electroplated surface under substantially nonoxidizingconditions to a temperature above the melting point of the electroplatedcoating.

8. A process for electroplating cold-rolled steel comprising treatingsaid steel in an acid solution until said steel is visibly etched,thereafter subjecting said metal to an anodic treatment in an alkalinecyanide solution, electroplating the treated steel with tin andsubsequently heating the electroplated steel under substantiallynon-oxidizing conditions to a temperature above the melting point of theelectroplated tin.

9. A process for electroplating steel comprising treating steel in anacidic solution until said steel is visibly etched, thereafter treatingsaid steel with an alkaline solution for a period of at least threeminutes, electroplating the treated steel with tin and subsequentlyheating the electroplated steel under substantially non-oxidizingconditions to a temperature above the melting point of the electroplawdtin.

10. A process for electroplating steel comprising treating steel in anacidic solution until said steel is visibly etched, thereafter treatingsaid steel with an alkaline solution containing alkali metal cyanide fora period of at least three minutes, electroplating the treated steelwith tin and subsequently heating the electroplated steel undersubstantially non-oxidizing conditions to a temperature above themelting point of the electroplated tin.

11. A process for electroplating cold-worked I steel comprising treatingsaid steel in an acidic solution until said steel is visibly etched,thereafter treating said steel with an alkaline solutionfor a period ofat least three minutes, electroplating the treated steel with tin andsubsequently heating the electroplated steel under substantiallynon-oxidizing conditions to a temperature above the melting point of theelectroplated tin.

12. A process for electroplating cold-worked steel comprising treatingsaid steel in an acidic solution until said steel is visibly etched,thereafter treating said steel with an alkaline solution containingalkali metal cyanide for a period of at least three minutes,electroplating the treated steel with tin and subsequently heating theelectroplated steel under substantially non-oxidiz-' ing conditions to atemperature above the melting point of the electroplated tin.

FLOYD F. OPLINGER.

